ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Acknowledgment
The progress described in this paper represents the combined efforts of the development, engineering, and operating divisions of the K-25 and Paducah plants of the Carbide and Carbon Chemicals Co. The authors, in reporting this work, are acting 8 8 the representatives of a group of men. I n addition to the authors, others who have made particularly significant contribu-
Rice, R. Thomas, B. H. Thompson, -4.Tuholsky, A. Varlan, C. 'VV. Walter, and J. L. Waters. Literature cited (1) Osborne, S. G., a n d Brandegee, M. M., IND.ENG.CHEM.,39, 273 (1947). RECEIVED for review November 10, 1954. ACCEPTED February 3, 1955. Presented before the Dlvision of Industrial and Engineerlng Chemistry,
Improved Mediurn Temperat ure + Fluorine Cell J. DYKSTRA,
s.
KATZ,
c.
B. CLIFFORD,
E.
w.
POWELL,AND G.
H. MONTILLON
Carbide and Carbon Chemicals C o . , Gaseous Diffusion Plants, Oak Ridge, Tenn., und Puducah, Ky.
B
ECAUSE of the large quantities of elemental fluorine required for processes of interest t o the Atomic Energy Commission program, considerable effort has been expended toward decreasing fluorine costs and improvement of fluorine cell operation. The progress achieved to date makes it apparent that general benefit to the chemical industry may result from a consideration of these improvements in fluorine manufacture. The fluorine cells used in the program are of the medium temperature type previously described (3-Y). The cells operate a t 100' to 105" C. using a fused salt bath of KF.2HF (melting point 71.7" C.). The fluorine is liberated a t a carbon anode and hydrogen is liberated at a steel cathode. The characteristics of the improved cell are compared in Table I with the characteristics of the Hooker cell ( 5 ) ; the improved cell resembles most closely the Hooker cell among those previously reviewed.
CARBIDE'S NEW FLUORINE CELL compared with the 7947 Hooker cell, has
. . . doubled cell operating current . . . doubled cell life . . . increased operating efficiency Cell construction features welded Monel tank with steel water jacket
The cell body consists of a welded Monel tank with a steel water jacket for cooling and heating (Figure 1). The original tank and water jacket were of steel. Considerable production experience was gained with tanks fabricated from low carbonlow silicon steel, 0.010-inch nickel electroplated steel, magnesium alloy, and hlonel; stress relieving is required on all welded fabrication. The following approximate service life was experienced: Ampere Hours Steel 12 x 1 0 6 kiigel-plated steel 20 x 1 0 6 Magnesium alloy (96% Mg, 3y0 Al, 1% Zn, 0.2% hln) 20 X IOB Monel >40 X 10'
Figure 1.
Cell body
The developments reported represent work done over a period of more than 5 years on a large scale continuous plant operation. T h e design features of cell construction and the cell operation procedures contributing to the improved characteristics and cell life are presented in that order. M a y 1955
Additional heat transfer capacity was provided by installation of baffles in the water jacket and water recirculation tubes in the center of the tank. Wheels were attached to the cell body to facilitate transfer during installation and removal of cells from the fluorine plant. The cell head (Figures 2, 3, 4) consists of a steel plate with fluorine and hydrogen compartment-separating skirts, externally threaded packing glands for support of anodes and cathodes, fluorine and hydrogen gas outlet pipes, nitrogen purge and hydrogen fluoride feed lines, electrolyte thermocouple wells, and electrolyte sample well. The cell head was a t one time welded to the
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Table 1.
Cell Characteristics Hooker Cell 2,000 24
Maximum current, amp. Effective anode area, sq. ft. Maximum anode current density, amw/sq. ft. 83 Size, inches Length 68 Width 32 Height 29 Anodes Number 28 Dimensions 1 . 2 5 X 6 . 2 5 X 18 Operating temp., ", C. 100-105 Hydrogen fluoride In electrolyte, % 39 Life, in thous. of ampere hours 2,600" Anodes 9,000-10, 100a Diaphragms 20,000-30, 1000 Cell bodies Estimated maximum. b Average of large numbes of cells over a 3-year period.
Improved Cell 4.000 32 125 68
32
30
24 1 . 2 5 X 8 X 18 100-105 41 5,000b 40,OOOb
>40,000 b
cell body after assembly of anodes, cathodes, and diaphragms, but currently it is attached with a bolted flange and sealed with a rubber gasket. Rubber gaskets when compressed, have proved satisfactory in fluorine service; the rubber exposed to fluorine apparently forms an organic fluorocarbon which expands and resists further penetration of fluorine. The fluorine and hydrogen compartment separators consist of two rectangular skirts welded t o the under side of the cover plate and projecting a minimum of 2 inches below the electrolyte level. Steel and Monel skirts have been used extensively with Monel having a life of approximately four times that of steel. The steel headplate has almost indefinite life. Welded magnesium alloys (96% Mg, 3% Al, 1% Zn, O , Z % Mn) and (98% Mg, 2 % Mn) headplates and skirts have also been tried experimentally to relieve skirt corrosion. These alloys appeared superior to steel as cell heads when operated with anode potential; however, steel and Monel skirt replacement is simpler and less expensive. Anode assembly gives 32 square feet of active surface per cell
The anode assembly consists of twelve carbon anodes arranged in two parallel rows of six each. Two assemblies totaling 24 anodes are used in each cell. The assemblies are each suspended a84
1.5 inches from the cathode by two copper rods brazed to the solid sup port bar and projected through packing glands in the cell head as shown in Figure 5. These copper suspension rods also serve as the electrical conductors. Flat carbon anodes 18 x 8 X 1.25 inches are used; the upper 2.5-inch contact surface is machined and drilled for four support bolts Twelve inches of the anode is electrochemically active thus giving 32 square feet of active anode surface area per cell or a current density of 125 amperes per square foot a t 4000ampere operation. The anodes are bolted between a solid bar and a separate contSct plate for each anode as shown in Figure 6 using 85-foot-pound torque on 0.75inch cap screws. The cap screw threads are lubricated with powdered graphite and c h l o r o t r if l u o r o e t h y l e n e liquid polymer. The use of the high torque assembly prevents electrolyte from seeping between the carbon and the support; this seepage would shorten the cell life by increasing 1he contact resistance. Under normal operating conditions, no increase in contact resistance is experienced during a 200-day life of some of the longer-lived cell assemblies. Anode supports of both solid and channel design have been used of steel, copper, nickel, and chromemolybdenum steel. Chrome-molybdenum steel ( AISI-4140) has proved to have a service life comparable to copper a t approximately one third the cost. Steel and nickel support bars corroded rapidly resulting in poor contact and shortened cell life. Solid anode support bars require a minimum of machining and thus cost considerably less than the channel-type support. The anode contact surface is machined to a scratch-free finish before each assembly. Operating experience with Monel, Everdur, copper, and several types of steel bolts and cap screws has been obtained. Most steels and Monel proved to be unsatisfactory because of a high dissolution rate. Everdur and copper proved unsatisfactory because of torque limitations and the resulting high contact resistance. AISI-4140 chrome-molybdenum steel cap screws are used for anode support; the cost is one fifth that of copper. Anodes. Carbon anodes of CAA and YAA grade are used with typical physical properties as shown in Table 11.
Table II. Grade GAA YAA
Physical Properties of Anodes Bulk Density, G./Cc. 1.55 1.70
Specific Resietance, Ohm-Inch 0.0017 0.0013
Flexural Strength, Lb./Sq. Inoh 3000 3100
No corrosive attack haA been observed on specification grade carbon anodes during continuous service periods of greater than 200 days although graphite anodes deteriorated rapidly. The selection of anodes appeared necessary because of the wide variation in anode current density observed on neighboring anodes in experimental cells. Attempts were made to avoid the possibility of varying the current-carrying capacity of anodw in the same cell by correlating the following properties: resonant frequency ( I ) , electrical resistivity, hardness, porosity, and spectrographic analysis. None of these single measurements appeared simple enough on a production basis to warrant its use
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
1
except the resonant frequency test which roughly correlates the rigidity of a n anode with its electrical properties, thermal properties, and hardness. While the evidence is not conclusive, more than 300 cells have been assembled and operated without start-up difficulty since this anode-selection program was initiated, and further efforts to correlate resonant frequency and cell performance are continuing. Cathodes. The cathode assembly consists of three vertical parallel steel plates welded (stress relieved) to cross plates at the ends and suspended by two 1-inch steel rods a t one end and one steel rod at the other end; two assemblies are needed for each cell. The suspension rods extend through an insulated packing gland in the cell cover and also serve as the electrical conductors. Electrical contact resistance is minimized by silver electroplating the area which is clamped to the electrical bus. Screen Diaphragms. The diaphragm assembly (Figure 4) consists of a rectangular Monel angle frame with the flange drilled at 8-inch centers for cap-screw support to the under surface of the gas-separation skirt of the cell head. Six-mesh woven Monel screen is welded to the angle frame with Monel reinforcement strips welded a t the ends and bottom of the screen to provide rigidity and permit spacing and alignment of the diaphragm equal distance between the anode and cathode. This spacing is carefully checked on each assembly, with necessary adjustments being made on the positioning of the anode or cathode assembly. Hydrogen evolved a t the cathode during cell operation passes through the electrolyte and is diverted by the Monel diaphragm frame attached to the gas-separation skirt into the hydrogen gas chamber of the cell head. Hydrogen evolved on the surfaces of the center cathode sheet is collected by a Mono1 inverted channel welded to the rectangular diaphragm frame and passes
HEAD
CARBON ANODE
SCREEN IRE M E S H )
ELECTRODE SUPPORTS
CATHODE
Figure
May 1955
4.
Cell head assembly
into the hydrogen gas chamber at either end of the frame. The diaphragms also serve as a barrier preventing broken carbon anodes from making direct contact between adjacent active anodes and the cathode thus “shorting out” the cell. The diaphragm frame is electrically insulated from the gas-separation skirt of the cell head with a Teflon gasket and bolted to the separation skirt with 0.25-inch Monel cap screws, which are electrically insulated from the Monel frame by a Teflon sleeve and washer. Corrosion of the ,Monel diaphragm described is negligible after a continuous service life of greater than one year. Steel diaphragm assemblies have also been used, but they had high corrosion rates.
LAMIGOID GASKET LAMIGOID RING
STEEL PRESSURE RING PACKING G L A N D NUT
LAMIGOID GASKE
PACKING G L A N D FlTTlNC
RUBBER GASKETS TEFLON GA
ELECTRODE
Figure
5.
Packing gland
Cell Cleaning and Drying. All cap screws, anode support bars, diaphragms, and gaskets are thoroughly cleaned and degreased with trichloroethylene before assembly. The entire cell including the anodes is then dried for 16 hours after assembly with an air purge a t 180” to 210’ F. before charging with electrolyte. Cell operates continuously at 3500 to 4000 amperes until failure
Electrolyte Preparation. Approximately 2000 pounds of electrolyte containing 41 weight yo hydrogen fluoride is: charged to each cell. Explosions and polarization of cells on start-up have been reduced by exacting raw material specifications and careful preparation of the electrolyte. Hydrogen fluoride impurities have been reduced to less than 0.1%. Potassium bifluoride sulfate ion impurity is less than 0.01%. All electrolyte is preconditioned by bubbling fluorine through the mixture before use; moisture content of the electrolyte thus prepared is 0.001 to 0.003%. Start-up Procedures. Two methods of cell start-up h a w been used. The favored method consists of start-up a t anode current densities of approximately 125 amperes per square foot. With this method, the cell normally will polarize, but continued operation for 2 to 10 minutes at the high current densities and 30 to 48 volts will usually result in either sudden depolarization during operation or depolarization after shutdown. The cell will then operate without difficulty a t 8 to 10 volts and anode current densities up to 125 amperes per square foot. The alternate method consists of a 48- to 72-hour period of operation at a n anode current density of 15 to 25 amperes per square foot. Unsuccessful depolarization techniques attempted were exposure of carbon anodes to fluorine or hydrogen fluoride before assembly, short period operation with reverse polarity, and alternating current. Normal Operation. The cell is normally operated a t 3500 to 4000 amperes continuously until failure. Current efficiency checks that have been made on individual cells indicate a current efficiency of 90 to 95% and a power efficiency of about 27y0 for 4000-ampere operation.
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Figure 6.
Anode assembly
Gaseous hydrogen fluoride is continuously fed to the electrolyte through a well line a t a rate based on operating amperage and checked by electrolyte level measurements and hydrogen fluoride analysis of the electrolyte. The possible range for operation a t 100 to 125 amperes per square foot lies between 39 and 43 weight % hydrogen fluoride in the electrolyte. At less than 39 weight yo the voltage increases, and polarization frequently occurs; above 43 weight %, hydrogen fluoride carry-over in the fluorine is excessive, and all metal parts corrode rapidly. Monel tanks, gas-separation skirts, and chrome-molybdenum steel anode supports and cap screws have been observed to corrode beyond use in less than one half million ampere hours operation in electrolyte with 45 weight per cent hydrogen fluoride. Forty to 42 weight % hydrogen fluoride has been selected as the optimum plant range. The hydrogen and fluorine gas chambers are purged with dry, oil-free nitrogen before start-up to remove air. A nitrogen purge is also used in these chambers and the hydrogen fluoride feed line before removal of the cell for maintenance to minimize exposure of personnel to the toxic fumes.
Operating at 4000 amperes and 9.5 volts, electrolyte 220" F., about 4700 B.t.u. per hour would be lost from the top of the cell by radiartion and convection leaving 86,100 B.t.u. to be removed by the cooling water. With an inlet water temperature of 120' F. and a water rate of 40.0 pounds per minute, the exit water temperature would be 156' F. and the estimated heat removal by the cooling water would be 88,800 B.t.u. per hour, which& adequate. Polarization. Cells assembled with selected carbon anodes and initially depolarized with the short-period high voltage depolarization technique described normally operate without polarization until failure. Occasionally, polarization occurs which is attributed to introduction of moisture to the electrolyte through the acid feed or gas headers; this polarization is treated by addition of 5 pounds of lithium fluoride to the electrolyte either as a dry powder or as a liquid after blending with electrolyte. The cell is operated for a period of approximately one hour at 500 amperes and then returned to service. If polarization continues, the high voltage depolarization technique is repeated. Cause of Cell Failure. The major cause of cell failure is broken anodes. Explosive recombination of hydrogen and fluorine or oxygen gases is believed to be the primary cause of broken anodes. These recombinations result from pressure surges inherent in the continuous fluorine utilization process and from line plugging by electrolyte-mist carry-over in the gas outlets of the cell. Cell operation with low electrolyte levels and corrosion leaks in the gas separation skirts also may result in explosive recombination of hydrogen and fluorine. It is also believed that oxygen remaining from incomplete purging or introduced through air leaks may a t times cause explosions. Occasional cell failures are experienced as a result of water leaks in the cooling tubes or water jacket of the tank. Cell failures resulting from sludge accumulation in the electrolyte have been virtually eliminated by use of Monel and chrome-molybdenum steel in cell assemblies. Present development i s aimed at improved life and efficiency
The major improvements achieved to date are selection of better metal alloys and methods of metal fabrication, the use of rubber gaskets, the high torque anode assembly, the selection of anodes, the high purity of the raw materials, the high voltage depolarization, and the improved heat transfer system.
Table 111. Amperes a000
3500 4000
Vults 8.8 9.2 9.5
Values of Over-all Coefficient, Water Rate, Lb./Min. 62.0 62.0 62.0
U
U at Inlet Water Temperatures of 1 0 5 O F. 120' F. 135' F. 19.3 . 21.4 23.5 27.4 22.5 23.9 25.7 28.5 31.3
Temperature control at 210-220' F. requires removal of about 90,800 6.t.u. per hour
I n the operation of a fluorine cell, the actual voltage exceeds the theoretical decomposition voltage (2.85) by several volts. I n normal operation, at 4000 amperes and 9.5 volts about 90,800 l3.t.u. must be removed per hour to maintain cell temperatures constant a t 210 to 220" F. The water jacket on the sides of the cell and the central cooling tubes have about 38 square feet of cooling surface available. The water flow rate may be 40 to 80 pound8 per minute with water inlet and outlet temperatures of 90" to 120' and 115" to 145" F., respectively. With the normal flow of cooling water, heat transfer on the water side as well as on the electrolyte side is largely b y natural convection. On the electrolyte side, mixing and circulation are increased by the stirring action caused by the evolution of both hydrogen and fluorine. Table I11 shows calculated values of the over-all heat transfer coefficient, U , based on actual operating data. The electrolyte temperature is aseumed to be 220' F. and the electrolyte concentration 41 % hydrogen fluoride.
886
Present development efforts are directed to decreasing the cost of the manufacture of fluorine b y decreasing maintenance and increasing cell operating efficiency, current capacity, and cell life. Modifications being tested which may simplify maintenance are the removal of the screen diaphragms separating the anode and cathode compartments and the use of external anode supports to allow replacement of individual anodes. Measurement of hydrogen evolution as an indication of fluorine production is being applied as a means of studying the effects of changes in cell design and operation on cell efficiency. Enlarging the cell unit and improving cell cooling will achieve increased current capacity. Cells about 50% larger than those described here have been placed in operation recently; handling the large cell during installation and removal in the existing plant limited the increase in size. Increased cooling capacity and, therefore, potentially larger
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 47, No. 5
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT current capacity for a given cell is possible through cathode cooling; the heat transfer coefficient of a cathode cooled test cell has been demonstrated to be larger by a factor of 2 to 3 . A longer cell life can be achieved by decreasing anode breakage, improving the cell cooling, and enlarging the cell unit for decreased current density. Methods under study for avoiding breakage of anodes are the use of 2-inch instead of 1.25-inch anodes, multiple fluorine outlets, and the use of flexible anode supports. The larger cells may, b y allowing operation at lower current density and with more heat exchange area, make for longer life. More uniform cooling may possibly increase cell life, since local overheating may be a cause of other cell faults; better distribution of cooling was achieved in a test cell with water-cooled cathodes.
are D. Correll, R. A. Ebel, J. Finley, J. A. Marshall, R.Paluzelle, T. Shapiro, hf. Schussler, B. H. Thompson, A. F. Vincent, and J. Whitternore. literature cited (1) Am. SOC.Testing Naterials, Standards 1949, Part 3 , Designation C215-47T,“Sonic LIethod-Modulus of Elasticity of Concrete
Specimens.” (2) Downing, R. C., Benning, A. F., Downing, F. B., McHarness,
R. C.. Richards. 14. K.. and Tomkowit. T. W.. IND.ENG. CHERI.,39, 259 (1947). ( 3 ) Fowler, R. D., Burford, W. B., 111, Anderson, H. C., Hamilton, J. M., Jr., and Weber, C. E., I b i d . , 39, 266 (1947). (4) Gall, J. F., and Miller, H. C., Ibid., 39,262 (1947). (5) I\lurrey, R. L., Osborne, S. G., and Kircher, 31. S., I b i d . , 39, 249 I1 947’1. ( 6 ) Pinkston, J. T., Jr., Ibid., 39, 255 (1947). \ - - - - / .
Acknowledgment
The progress described here represents the combined efforts of the development, engineering, and operating divisions of the K-25 and Paducah plants of the Carbide and Carbon Chemicals Co. The authors, in reporting this work, are acting as the representatives of a group of men. I n addition to the authors, others who have made particularly significant contributions to the work
(7) Schumb, W. C., Young, R. C., and Radimer, K. J., Ibid., 39, 244 (1947). RECEIVED for review November 10, 1954. ACCEPTED February 3, 1955. Presented before the Division of Industrial and Engineering Chemistry a t t h e 126th Meeting of the A M E R I C A N CHEhfrcAL SOCIIETY, New York, N. Y.. September 1954. This document is based on work performed for the Atomic Energy Commission by Union Carbide and Carbon Corp. a t Oak Ridge, Tenn., and Paducah, Ky.
Purification of Gases for Ammania Removal of Carbon Monoxide by Cuprammonium Carbonate Solutions R. EGAl.ON, R. VANHILLE,
AND
M. WILLEMYNS
Efablissements Kuhlmann, Paris, France
A
S a result of their studies, Patry and Duguet (6) claimed that
the mechanism of the absorption of carbon monoxide b y cuprammonium carbonates is not known with certainty. Pavlov and Lopatin ( 7 ) reported t h a t there are almost no data concerning the effect of the chemical constituents of the absorbing solution. However, the importance of the removal of the last traces of carbon monoxide from the gas in the manufacture of ammonia from water gas is well known. I n order t o maintain the activity of the catalyst it is general practice in industry t o limit the allowed maximum proportion of carbon monoxide in the purified gas mixture ( Nz 3H2) under normal working conditions to less than 10 p.p.m. The space velocity of those gases through the catalyst and the production capacity of the plant are directly related to the strict control of the removal of carbon monoxide. T h e purification process may be regarded as a cycle of two phafies: 1. Absorption of carbon monoxide by cuprammonium carbonate (or formate) solution in one or several towers-formation of a cuprammonium-carbon monoxide complex 2. Thermal regeneration of the solution-destruction of the com lex at high temperature to liberate almost t h e total amount of agsorbed carbon monoxide and t o allow t h e recycling of t h e regenerated absorbing solution
+
The control of the gas purification by copper liquors is critical because of the high accuracy required and the numerous factors involved. T h e speed of the chemical processing of the cuprammonium carbonate solutions currently used in industry and per-
May 1955
manent contrgl of their constituents-Cu+, Cu *+, carbon dioxide, total ammonia, and “active” ammonia-present problems. The selection and control of the temperature of the thermal regeneration are also critical factors. This temperature ought t o be high in order to eliminate the maximum amount of the dissolved carbon monoxide and to avoid the permanent enrichment of the solution with carbon dioxide. (The converted gas contains, in fact, approximately 0.5 to 1% of carbon dioxide even after washing b y water under pressure.) Unfortunately, this temperature is strictly limited t o 80” C. hbove this temperature the depletion of the ammonia in the solution is excessive and causes the precipitation of copper from the cnprammonium salta and a t the same time a marked decrease in the absorbing power of the solution. Finally, the rational design of a plant for the purification of these gases b y copper liquors as well as the regulation of its normal operating conditions require quantitative knowledge of the influence of the variations of each of the constituents in the liquor on the absorbing power of the liquor, over the whole scale of temperatures and pressures found in practice. Serious studies of this problem published since 1940 in Germany, Russia and, more recently, in the United States, do not yield more than empirical, incomplete, and often contradictory results, and do not allow plant cbalculations or elucidate the anomalies and discontinuities often reported in industry. Zhavoronkov and coworkers (8) first published a mathematical relationship, identical with that for absorption isotherms by
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